Underground drilling, such as gas, oil, or geothermal drilling, generally involves drilling a wellbore through a formation deep in the earth. Such bores are formed by connecting a drill bit to long sections of pipe, referred to as a “drill pipe,” so as to form an assembly commonly referred to as a “drill string.” The drill string extends from the surface, to the bottom of the bore. The drill bit is rotated so that the drill bit advances into the earth, thereby forming the bore. In rotary drilling, the drill bit is rotated by rotating the drill string at the surface. Torque required to rotate the drill bit is generated above-ground, and is transferred to the drill bit by way of the drill string. Typical drilling systems use drilling mud pumped from the surface at high pressures through an internal passage in the drill string, and out through the drill bit. The drilling mud lubricates the drill bit and flushes cuttings from the path of the drill bit. The drilling mud then flows to the surface through an annular passage formed between the drill string and the surface of the bore. Drilling mud flowing through the internal passage can also be used to rotate the drill bit using what is referred as a mud motor. The mud motor is usually mounted in the drill string near the drill bit. The drill bit can be rotated by the mud motor alone, or by rotating the drill string while operating the mud motor.
Directional drilling refers to drilling systems configured to allow the drilling operator to direct the drill bit in a particular direction to reach a desired target that is located some distance vertically below the surface location of the drill rig and is also offset some distance horizontally from the surface location of the drill rig. Steerable systems use steering tools, such as bent tools, located downhole for directional drilling and are designed direct the drill bit in the direction of the bend. Rotary steerable (RS) systems use moveable arms that can be directed against the borehole wall as the drill string rotates to cause directional change of the drill bit. Finally, rotatory steerable motor (RSM) systems also use moveable arms that can be directed against the borehole wall to guide the drill bit. The more recently developed rotary steerable motor systems, unlike rotary steerable systems, use the downhole motor to operate the moveable arms and rotate the drill bit with or without drill string rotation. RS and RSM systems may help operators drill vertical wells with less tortuosity.
Many drilling systems today include instrumentation modules located downhole in the drill string. These modules includes various sensors used to monitor relevant properties of the geological formation, direction and orientation of the various components of the drill string, and sometimes certain operational parameters, such as motor speed, WOB, vibration, etc. Based on the information obtained from the sensors, the drilling operator controls the drilling operation, and may in some cases elect to guide the drill string in a particular direction. In other words, rather than following a predetermined trajectory, the trajectory of the drill string can be adjusted in response to the properties of the underground formations encountered during the drilling operation using a technique referred to as “geosteering.”
Boreholes deviate from the planned direction due to many factors, one of which is discontinuities in the formation being drilled due to fractures, canted formations, or other anomalies. These formation anomalies can apply a load to the drill bit which forces the drill bit off course. Drilling systems that include RS or RSM systems are used to apply forces on the drill string to reposition the drill bit in order bring the borehole back on course. Typically instrumentation modules are located some distance back, or uphole, relative to drill bit. At times, the instrumentation module may as much as 60 feet uphole relative to the drill bit. Thus, during any deviation in drilling direction due to formation anomalies is not detected until the instrumentation module is aligned with onset of the borehole deviation. And while directional drilling systems, with or without drilling operator analysis and intervention, may be used to control the direction of the drill bit when a deviation is observed, the distance between the drill bit and the instrumentation module can create a lag-time between the time the deviation is observed and the time that corrective action is implemented. Directional drilling systems help control drilling direction but may not reduce all borehole deviations.